Five Methods for Enhancing Color and Clarity in Rubies and Sapphires
In the world of gemstones, corundum – the family that includes rubies and sapphires – is a favourite among jewellers and collectors alike. Yet, many may not be aware that these stunning gems often undergo treatments to enhance their colour and clarity.
Heat treatment and diffusion are the two most common methods used in the transformation of corundum. Heat treatment, the oldest method, involves heating the gemstone in either an oxidizing or reducing atmosphere without adding any foreign substances. This process can lighten or darken the colour, improve clarity, and even produce optical phenomena like asterism.
However, the most dramatic changes are achieved through heat treatment with additives, also known as diffusion. This process involves heating the corundum in the presence of certain impurities such as beryllium, titanium, iron, chromium, nickel, or borax-based substances. These additives diffuse into the crystal lattice, altering or enhancing the colour more dramatically.
One example of diffusion is the colour diffusion of chromium, which produces pink to red stones, while nickel is used for yellow ones. Titanium diffusion can improve a star sapphire's optical effect by causing needle-like crystals to grow. Evidence of the former presence of needle-like inclusions may remain as lines of tiny dot-like inclusions or small colour patches along the directions of the inclusions.
Another treatment method is fracture filling with lead glass, particularly for rubies. This technique improves transparency and appearance, especially for large stones, but it also makes the stone more fragile and easily detected due to the different physical properties of the filler.
Gemmologists play a crucial role in identifying these treatments. They use microscopic examination to detect alterations in internal inclusions, characteristic inclusions or "heat halos," and the presence of a surface diffusion layer. Advanced methods like UV-Vis or Raman spectroscopy can detect the presence of diffused elements or changes in the chemical structure indicative of treatment. Refractometer and specific gravity tests can help identify glass-filled rubies due to altered physical properties compared to natural corundum.
These treatments have been a part of corundum's history for over 1,000 years. Early treatments included dyeing and heating using charcoal fires and blowpipes. Today, more sophisticated treatment methods are used, as seen in the glass-filled rubies from Mozambique and stones from Mong Hsu in Myanmar that are partly repaired through heat treatment with a flux like borax.
As the gem and jewellery trades continue to evolve, it is essential for professionals to stay informed about developments in gemmology, pay attention to ways treated stones may be identified, and consider sending doubtful stones to laboratories to confirm their status. By doing so, they can ensure that their customers receive genuine, high-quality corundum gemstones.
[1] GIA (Gemological Institute of America). (n.d.). Heat Treatment of Gemstones. Retrieved from https://www.gia.edu/gem-identification/heat-treatment [2] AGTA (American Gem Trade Association). (n.d.). Ruby. Retrieved from https://www.agta.org/gemstone-encyclopedia/ruby/ [3] AGTA (American Gem Trade Association). (n.d.). Sapphire. Retrieved from https://www.agta.org/gemstone-encyclopedia/sapphire/ [4] AGTA (American Gem Trade Association). (n.d.). Glass-Filled Rubies. Retrieved from https://www.agta.org/gemstone-encyclopedia/glass-filled-rubies/ [5] GIA (Gemological Institute of America). (n.d.). Lead Glass-Filled Rubies. Retrieved from https://www.gia.edu/gem-identification/lead-glass-filled-rubies
- In the realm of gemmology, workshops can provide valuable insights into various treatment methods for enhancing the color and clarity of corundum gemstones, such as heat treatment, diffusion, and fracture filling with lead glass.
- Beyond the world of jewellery and lifestyle, the science of gemmology is crucial in identifying medical-conditions related to corundum, including changes in internal inclusions, characteristic inclusions, or "heat halos," that may be indicative of certain treatments.
- With advancements in technology, gemmologists can now employ modern techniques like UV-Vis or Raman spectroscopy to detect the presence of diffused elements or changes in the chemical structure, ensuring the authenticity and quality of corundum in the age of increasingly sophisticated treatments in gemmology.
